1. Field of the Invention
The present invention relates to a magnetic recording medium which is used with a video recorder fitted for a high density recording, in particular a thin type magnetic recording medium which copes with the prolongation of a recording time. More particularly, the present invention relates to a magnetic recording medium comprising a non-magnetic support, a magnetic layer on one surface of the non-magnetic support, a back coating layer on the other surface of the non-magnetic support and a primer layer containing acicular (needle-form) hexagonal ferrite magnetic powder between the non-magnetic support and at least one of the magnetic layer and the back coating layer.
2. Description of the Related Art
Recently, with the increase of requirements on dezitization and a high recording density such as a large recording capacity, diminishing of a wavelength of a recording signal and decrease of a total tape thickness due to increase of recorded informations are highly desired with a magnetic recording medium, in particular a magnetic recording tape. But, decrease of the total thickness of the magnetic recording tape deteriorates running properties and durability of the tape considerably and is disadvantageous for the electromagnetic conversion characteristics.
Such phenomena are induced by large decrease of rigidity of the tape which is caused by the decrease of the total tape thickness. A reason why the decrease of the tape rigidity influences the running properties and durability of the tape and also the electromagnetic conversion characteristics will be explained by making reference to a video tape.
The video tape runs while being wound around various types of tape guide posts of a VTR equipment at certain angles.
The tape guide posts include an upper side regulating one and a lower side regulating one for regulating a position of the running tape. When the running tape tends to depart from a position regulating post, a force for inhibiting the departure is generated by rigidity of the tape itself, namely a resiliency against an external force which will deform the tape. The tape rigidity acts as a force which controls contacting properties between the tape on which a constant tension is applied during running and a magnetic head, namely a spacing between the tape surface and the magnetic head and contacting stability between them.
Since the above inhibiting force is weakened by the decrease of the tape rigidity caused by the reduction of the total tape thickness, the tape is folded or the tape edges are deformed in a wave form, and further, among the electromagnetic conversion characteristics, readout output decreases and output variation increases. As a method for suppressing the decrease of the rigidity of the tape which causes a problem in the thin video tape having, for example, a thickness of 12 .mu.m in the VHS format and 10 .mu.m in an 8 mm video format, an increase in the rigidity of each of the non-magnetic support, the magnetic recording layer and the back coating layer has been studied.
To increase the rigidity of the non-magnetic support, it has been studied to use a highly rigid high heat resistant material such as an aromatic polyamide and an aromatic polyimide in place of polyethylene terephthalate (hereinafter referred to as "PET") or polyethylene naphthalate (hereinafter referred to as "PEN") which is conventionally used as the non-magnetic support (see Japanese Patent Publication Nos. 3121/1974 and 43364/1989).
While the rigidity of the proposed materials is high, not only are the materials are more expensive than PET or PEN, but their adhesion with the magnetic layer or the back coating layer is poor and they have high moisture absorbance.
Another method for increasing the rigidity of the tape includes the formation of a thin metal layer having high rigidity between the magnetic layer and the non-magnetic support, which is practically used (see Japanese Patent Kokai Publication Nos. 143433/1985, 205821/1985 and 29318/1988).
Since, in the above methods, the adhesion of the magnetic layer or the back coating layer to the thin metal layer greatly decreases or there is a difference in rigidity between the layers, some problems arise, for example, in a tape slitting step.
To increase the rigidity of the magnetic layer or the back coating layer, it has been proposed to increase the content of a filler such as magnetic powder in the magnetic layer, add a reinforcing filler to the layer, or increase the glass transition temperature of a binder resin (see Japanese Patent Publication Nos. 29334/1988 and 26285/1990 and Japanese Patent Kokai Publication No. 278020/1986).
Increase of the content of the magnetic powder in the magnetic layer worsens the durability of the tape. The addition of the reinforcing filler does not improve dispersibility or orientation of the filler but spoils surface smoothness. When the glass transition temperature of the binder resin is increased, the rigidity of the coated layer cannot be increased greatly. Rather, since the binder resin becomes hard, the surface smoothening is prevented, and it is difficult to satisfy the dispersibility and reactivity with other additives in the coated layer and durability at the same time.
As explained above, in connection with the surface smoothening and the decrease of the thickness which are required for the magnetic recording medium so as to comply with the increase of recording density and the recording capacity, it is very difficult to achieve satisfactory running properties and durability while maintaining the electromagnetic conversion characteristics unchanged.